This invention relates to flip chip integrated circuit (IC) packages and more specifically to multi-chip IC packages with high interconnect density.
Multi-chip IC packages are becoming standard in the IC industry because of increased device packing density, which translates into smaller packages with reduced cost. Typical multi-chip packages have multiple chips or multi-chip modules carried by a printed wiring board. In an effort to increase device packing density, IC packages have been proposed and used with multiple chips mounted on both sides of a printed wiring board. This approach complicates the interconnection strategy in the overall assembly, but reduces by nearly one-half the footprint of the IC package. More recent developments in high density, low cost, IC packaging have taken advantage of flexible substrates which in some package designs allow more versatility for final assembly of the system. Flexible substrates are also used increasingly due to very fine, and thus dense, interconnection patterns. Also, flexible substrates permit simple through hole interconnections. It has also been proposed to populate both sides of a flexible substrate with active devices. Thin interconnect substrates that have through hole interconnections may be used to advantage to reduce the interconnection length between critical device components. Thinner substrates produce shorter interconnections. However, there is a limit to reducing substrate thickness and still have a robust IC package. It has been proposed to bond flexible substrates to rigid substrates to provide a sturdy package with added interconnect levels, but this eliminates the option of having devices mounted on both sides of the flexible substrate.
We have developed a high density multi-chip package using ultra thin flexible substrates with active chips on both sides of the flexible substrate. In the preferred embodiment, logic chips are mounted on one side of the flexible substrate and memory chips on the other side. The thin flexible substrate is provided with mechanical support by bonding the flexible substrate to a rigid support substrate, e.g., a metal sheet or a printed circuit board. Openings in the support substrate are provided to accommodate the IC chips on the side of the flexible substrate that is bonded to the rigid board. Interconnections between the logic and memory chips are made using vias, and the vias are exceptionally short due to the very thin flexible substrate. This arrangement also allows efficient heat sinking of the IC chips in the recessed portion of the rigid board by covering the recessed portion with a heat sink plate.